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Water hardness is measured (at least in the U.S.) in grains per gallon of water. Presumably the company that installed your water softener tested the water, and wrote the hardness down on the softener somewhere. So, let's take an example.

Suppose your water is 10 grains per gallon hardness. And suppose there are four people in your house. Each person uses approximately 70 gallons of water per day, so the total number of grains of hardness per day would be 10 x 4 x 70 = 2,800 grains per day. Ideally, you would like your softener to regenerate approximately every 7 days, therefore the softener would need to handle 2,800 x 7 = 19,600 grains per seven days.

Based on your water softener table, you would need to add approximately 10 lbs of salt every seven days, and set the timer to regenerate every seven days. Of course, you may have an automatic system that senses water hardness and automatically regenerates as needed. You will have to consult your manual, or else contact the installer, to determine exactly how to set up your system. Note that the table you showed only indicates CAPACITY of the water softener, it does not say anything about the actual hardness of your water.

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FYI our grains per gallon is 22 in Waterloo On Canada - very high. There are 4 people in the house however our last water softener tracked water usage over the past number of years and we're sitting at 135 gallons per day. I guess I should get the shower fixed :-)

With those numbers above then we're using 22 x 135 = 2970 grains / day? And in a perfect world the softener (which does re-generate on demand) should re-gen every 18,800/2970 = approx. 6 days using approx 10 lbs of salt?

Only fyi I installed the softener myself - it's a GE Model GXSF18G... might be a little small when our kids hit the teen years.

Most softeners need a 24 hr reserve capacity so you don't run out of capacity before the next time of regeneration IF the meter counts down to 0 gallons remaining. So you may have to subtract a day's capacity from the meter setting gallons if the control valve doesn't calculate a reserve automatically.

Correct. At 70 gallons per person per day, you're @ 7700 grains per day. Divide 7700 into the rated capacity to get the number of days. However, as Gary has pointed out, softeners keep a reserve; you need to drop the total capacity by 20-25% for GE, to mitigate the reserve.
See the following snippet from the GE website. http://www.geappliances.com/search/f...e/10000268.htm

The interesting thing here is that they state upfront that their softeners will regenerate every 3 to 6 days. If I reverse engineer that statement it implies that the softener will use as much or as little salt is required based on what the total water usage has been from the last regen, 3 to 6 days out.

For my softener, at 18,800 capacity, with 25% reserve (worse case), I end up with a 14,100 grain system. At roughly 3000 grains per day (22 gpg * 135 gallons), it should regen every 4 to 5 days (14,100/3000) using 10 lbs of salt. It keeps stats and modifies its programming over time so I will have a better idea of actual operation in a month or 2.

By the way, this unit is providing MUCH better quality water than the 39K Kenmore Genius II softener I had, and is much more efficient. I will monitor regen frequency / salt usage to make sure it's somewhere in spec.